The present invention relates to an improved process for the preparation of arylphosphinic acids. More particularly, the present invention relates to a process for preparing arylphosphinic acids by an aluminum chloride catalyzed reaction of an aromatic compound with phosphorus trichloride in which the prior art difficulties associated with separating the product from aluminum chloride complexes are effectively solved.
Arylphosphinic acids are compounds which are useful as catalysts and stabilizers in nylon synthesis and are also useful as intermediates in the production of a wide variety of useful compositions, such as pesticides, fuel and oil additives and the like.
One of the best known methods for preparing these compounds involves the reaction of an aromatic compound with phosphorus trichloride in the presence of aluminum trichloride to produce an aryl phosphonous dichloride, which is then hydrolyzed to produce the acid. Unfortunately, however, the aluminum chloride forms certain complexes during the process, which makes recovery of the final product difficult.
A method suggested by the prior art for circumventing the problems presented by the aluminum chloride complexes involves the addition of phosphorus oxychloride to the reaction mixture to form an aluminum chloride-phosphorus oxychloride complex, which settles from the reaction mixture, thereby facilitating recovery of the aryl phosphonous dichloride product. While this method is helpful, it is less than desirable because large amounts of phosphorus oxychloride are required and the remaining aluminum chloride-phosphorus oxychloride precipitate is a reactive waste product which can be difficult to dispose of. In addition, not all of the aluminum complex is removed, and the small amount remaining can lead to the formation of an emulsion during a subsequent reaction of the aryl phosphonous dichloride with water, which further complicates the process.
U.S. Pat. No. 3,974,217 teaches the preparation of alkoxy and alkylthio substituted phenyl phosphonous dichlorides by reacting an appropriate substituted aromatic compound with phosphorus trichloride in the presence of stannic chloride or titanium tetrachloride. It would appear, however, that this method would be less than successful with compounds not having the activating alkoxy or alkylthio substituents, since the catalysts used are less effective Friedel-Crafts catalysts.
A need therefore exists for a method by which arylphosphinic acids can be prepared from appropriate aromatic compounds and phosphorus trichloride using aluminum chloride as catalyst without encountering the prior art difficulties occasioned by the formation of aluminum chloride complexes.